1. Field of the Invention
This invention relates to a process for producing hydraulically setting extruding materials reinforced with polyvinyl alcohol fibers (hereinafter referred to as PVA fibers).
2. Description of the Prior Art
In recent years, extruding hydraulically setting materials reinforced with asbestos fibers show a remarkable growth mainly as interior and exterior construction materials in virtue of not only their excellent physical properties such as high degree of strength, and light-weightness, sound insulating properties, heat retaining properties etc. resulting from the blow molding but also their easy workability.
Hydraulically setting materials such as cement etc. have features of high compression strength, excellent inflammability and durability and inexpensiveness but suffer from disadvantages that the tensile strength, bending strength and impact strength are low and further that they tend to easily crack and are poor in dimentional stability.
Therefore, in order to offset the above-described drawbacks and improve the molding properties and shape retaining properties of said extruding hydraulically setting materials, asbestos fibers are generally employed. However, asbestos fibers have lately been strongly adversely pointed out for their harmness to human health, and many leading countries in the world have set severe regulations on their use which seem to become even severer. Among those, even some countries have already totally banned their industrial utilization. Also in Japan, this is designated as a specific chemical substance by the Industrial Safety and Hygiene Law, and there are various restrictions on their actual use.
Under such circumstances, reinforcing fibers which can replace the asbestos fibers have been eagarly sought, and many studies have been made, yet no product has been materialized in the field of extruding hydraulically setting materials. One main reason for this is that it is impossible to uniformly disperse fibers in extruding materials.
A general process for producing extruding materials comprises adding a small amount of a tackifier (e.g. methyl cellulose etc.) to a matrix material chiefly comprising a hydraulically setting material such as cement to impart viscosity, further adding 20-30% by weight of asbestos fibers of a grade with a shorter fiber length to impart moldability and reinforcement, mixing under dry conditions, thereafter adding a small amount of water, kneading the mixture in a screw-type kneader, connecting with an extruder and extruding through a die to obtain an extruded product.
In hydraulically setting materials such as asbestos slates etc., which are molded by the so-called wet type sheet making process, organic fibers such as Vinylon, polypropylene, nylon etc. have already been commercially employed as substitutes or supplements for the asbestos fibers. In such a case, a matrix material and fibers are mixed and dispersed together as a dilute slurry (50-200 g/l), then dipped up on a round or rectangular screen, and excess water is removed by a press to obtain a product. Since the fibers are dispersed in a dilute slurry, a uniform dispersion may be easily obtained.
On the other hand, in the case of extruding materials, the amount of water is restricted from an aspect of the shape retaining properties. In other words, in order to uniformly extrude from a die, it is preferred to be more flowable, that is, abundant in water, whereas in order to retain the extruded shape as it is (i.e. shape retaining properties), less flowability is more preferred. Further in the case of the present method, since it is impossible to take a step of forcedly removing the water content after extruding, it is preferred to reduce the amount of water added (i.e. reduce the ratio of water-hydraulically setting material) commonly recognized in the art. In order to solve such a problem, it is practiced to make a highly viscous matrix having flowability and also good shape retaining properties by employing a reduced amount of water and further employing an appropriate amount of an organic polymer such as methyl cellulose etc. as a tackifier. However, the dispersion of the fibers in such a highly viscous matrix is remarkably difficult, and no satisfactory extruded product is obtained. Even the organic fibers employed in the above-mentioned wet type sheet making process cannot be uniformly dispersed in extruding materials.
On the other hand, among inorganic fibers, alkali-resistant glass fibers are being under study. However, said fibers are brittle in nature and have a drawback of being very fragile, and therefore, as a countermeasture, they are used as collected yarns but still are seriously damaged by the shear force by a screw on kneading and extruding in the extruding step, and no reinforcing effect is obtained ("Shin-Kenchiku Doboku Fukugo Zairyo", p. 443, published from Joho Kaihatsu-sha, October, 1979). In addition, it is pointed out that their alkali resistance is not adequate, and accordingly their durability is doubted because most hydraulically setting materials exhibit alkalinity.
Steel fibers are not usable, not only because they are poor in flexibility and hence poor in moldability, for example, they tend to bend by the shear force during the extruding step, they cannot be smoothly extruded etc., but also because they remarkably deteriorate the appearance of the final products.
In spite of the fact that the replacement of the asbestos fibers has been eagarly sought as described above, there are no substitutes suitable for extruding materials, and the present situation is such that asbestos fibers are inevitably employed in such large amounts as 20-30%. Under such circumstances, the present inventors have been intensively studying on the replacement of the asbestos fibers and have come to the present invention.
Although the reinforcing mechanism of a brittle substance by a fibrous material is not simple, it may be typically considered as follows: that is, the reinforcing mechanism is a problem between the bearing of the stress by the fibers and the reinforcing efficiency. As for the former factor, when an external stress, such as that represented by pulling, is applied to a composite reinforced by a shortly-cut fibrous material, the stress imposed on the total composite is ideally expressed by ##EQU1## and even after fine cracks have been generated in the brittle matrix, the member of the left effectively functions and the strength as the total composite is enhanced (Hannant). More specifically, using 1 for the fiber length of short fibers, Lc for the critical fiber length, .sigma.fu for the breaking strength of the fibers and Vf for the volume fraction taken as the amount added, then the reinforcing effect by the fibers .sigma.R as regards pulling may be expressed as ##EQU2## wherein .eta. is the orientation coefficient of the fibers.
On the other hand, taking .tau. for the interfacial bonding force between the fibers and the matrix and d for the diameter of the fibers by presuming the fibers in the cylindrical form, then the critical fiber length has the following relationship: ##EQU3## then the equation (1) may be converted into ##EQU4## To summarize, in order to enhance the reinforcing effect by the fibers, it is required (a) that the strength should be high, (b) that the interfacial bonding power (.tau.) with the matrix should be high, (c) that from a morphological aspect of the fibers, the ratio 1/d (hereinafter referred to as the aspect ratio) should be great, that is, the fibers should be thin and long, and (d) that the volume fraction of the fibers should be great. As regards the requisites (c) and (d), they may be freely chosen as long as man-made fibers are used. Therefore, if the fibers can be uniformly dispersed in the matrix, the reinforcing effect by the fibers is now governed by the aforesaid (a) strength of the fibers and (b) interfacial bonding power with the matrix.
As can be seen from that their primary use is in the industral material field, PVA fibers are highly strong. Also regarding the interfacial bonding power, since the chemical interfacial bonding power is high because PVA molecule chains contain hydroxyl groups having strong affinity to hydraulically setting materials and also since there are numerous small ribs on the fiber surface because the wet or dry spinning was effected and a high degree of drawing was imparted, they have a feature that the resistance to pulling in the matrix is high, that is, the physical interfacial bonding power is also high.
On the other hand, since those generally known as highly strong fibers, for example, organic fibers such as aramides, polyesters, nylon etc., carbon fibers and glass fibers, have poor bonding power, slip-off is brought about and the strength of the fiber per se cannot be effectively utilized, and as a result, the reinforcing effect is small. In addition, since the polyester and glass fibers are remarkably low in alkali resistance, they will give rise to a problem concerning durability if used in hydraulically setting materials which exhibit alkalinity, and the carbon fibers and aramides represented by Kevlar etc. are extremely expensive.
The PVA fibers are not only highly strong and good in bonding power with a hydraulically setting matrix, as hereinabove described, but also excellent in alkali resistance and weather resistance and further comparatively inexpensive. Thus, for reinforcing a hydraulically setting brittle matrix such as cement etc., the PVA fibers are most suitable. Therefore, it is believed that these are the most suitable materials as substitutes for asbestos fibers in asbestos fiber-reinforced extruding materials. However, investigations on the use of PVA fibers as reinforcing materials in extruding materials have been hardly made, and of course they have never been used industrially.
As described hereinabove, the reason for why they have hardly been studied in the extruding material field in spite of that they have excellent physical properties as reinforcing materials is entirely due to that it has been impossible to uniformly disperse them. The aforesaid equation (3) can only been established when the fibers have been uniformly dispersed in the matrix, and it is needless to say that if the dispersed condition is poor, the reinforcing effect is gravely impaired. Dispersion of the fibers is governed extensively by the aspect ratio. In other words, when the aspect ratio is lessened, the dispersion tends to be improved, but, as can be understood from the equation (3), the intended reinforcing effect is reduced. On the other hand, if the aspect ratio is increased, the intended reinforcing properties tend to become great, but the reinforcing properties would be reduced due to the aggravation of the dispersion and thus the actual reinforcing effect sometimes begins to decrease. Further, poor dispersion deteriorates the appearance, which leads to the loss of the commercial value. In the extruding process, which is different from the wet type sheet making process which, as described hereinabove, involves a dilute slurry by using a large amount of water, the amount of water added to a hydraulically setting matrix such as cement etc. is extremely small, and further dispersion of fibers is effected by e.g. a screw which has a relatively low rotational speed in the kneading and extruding steps, and therefore it is very difficult for such a process to disperse the fibers. Asbestos fibers are considered very good in dispersibility in a hydraulically setting material and this is one of the reasons for their extended use for reinforcing hydraulically setting materials coupled with their excellent fiber physical properties. Even with said asbestos fibers, for that reason that those presently employed for extrusion cannot be dispersed, Classes 6 and 7 which are in the form of powder having short fiber lengths are merely employed, from which it is easily recognized how difficult the uniform dispersion of the fibers in the extrusion process is. Said asbestos fibers have only a low reinforcing effect because of their powder form, and therefore under the present situation, the use of such large amounts as 20-30% is inevitable.
As described hereinabove, it is a common knowledge in the art that on producing an extruding material, fibers having a reinforcing effect, which are thin and long, i.e. which have a great aspect ratio, cannot be uniformly dispersed. Under such circumstances, the present inventors have been studying in order to uniformly disperse PVA fibers having basically excellent reinforcing characteristics and having a great aspect ratio in an extruding material, and have finally come to the present invention.